1. Field of the Invention
The present invention relates to a multiple-channel all-optical TDM Time-Division-Multiplexed-WDM(Wavelength Division Multiplexed) converter and a multiple-channel all-optical TDM demultiplexer which separate the time-division-multiplexed optical signal pulse train to every channel spatially and simultaneously, in a TDM optical transmission system.
2. Description of the Related Art
Research has been conducted extensively in time-division-multiplexed optical transmission systems to develop a TDM demultiplexer, which separates a time-division-multiplexed optical signal pulse train to every channel at high speed without converting the optical signal into an electric signal, as well as a TDM multiplexer.
FIG. 14 shows the configuration of a future time-division-multiplexed optical transmission system in which all the processing is performed all-optically such as the TDM demultiplexer. In the time-division-multiplexed optical transmission system shown in FIG. 14, each light that is output from N optical sources L-1 to L-N (N is an integer greater than 1), is modulated with the signal pulse trains of frequency f.sub.0 (an electric signal) in modulators M-1 to M-N arranged corresponding to optical sources L-1 to L-N.
Optical signal pulse trains output from modulators M-1 to M-N are multiplexed in time series in the optical multiplexer (MUX) and become a time-division-multiplexed optical signal pulse train at a bit rate of Nf.sub.0 bit/s over hundred Gbit/s. The time-division-multiplexed optical signal pulse train is transmitted in the transmission line consisting of optical fiber and optical repeaters devices (optical amplifiers) linearly or nonlinearly (soliton transmission), and is separated into the original N optical signal pulse trains at f.sub.0 bit/s spatially by the optical demultiplexer (DEMUX). Optical signal pulse trains separated by the optical DEMUX are detected by the N optical detection devices D-1 to D-N.
In addition to the above, the optical sampling circuit for monitoring the waveform of the time-division-multiplexed optical signal pulse train is positioned in front of the optical DEMUX. The bit rate of each optical signal pulse train separated spatially by the optical DEMUX becomes a bit rate of f.sub.0. In the TDM demultiplexer, the demultiplexing operation is performed all-optically in order to achieve a transmission bit rate of more than 100 G bits/s.
In future optical communication networks, TDM-WDM converters to convert TDM optical signal pulse trains into WDM optical signal pulse trains are needed.
FIG. 15 shows the role of the TDM-WDM converter in the above situation. The TDM-WDM converter shown in FIG. 15 is arranged in the node that converts a TDM optical signal pulse train into a WDM optical signal pulse train.
When the TDM optical signal pulse train of wavelength .lambda..sub.0 is input, the TDM-WDM converter converts the TDM optical signal pulse trains into optical signal pulse trains having different wavelengths .lambda..sub.1 to .lambda..sub.4, and outputs these WDM time-division-multiplexed optical signal pulse trains. Because the TDM demultiplexer and the TDM-WDM converter mentioned above are constituted on the basis of the same principal as the present invention, only the TDM demultiplexer (optical pulse demultiplexer) will be explained here.
FIG. 16 shows an example of a conventional optical pulse demultiplexer. The device shown in FIG. 16 is called a "nonlinear loop mirror". An optical Kerr medium (nonlinear optical material) 42 is inserted in a loop between two ports 41b and 41c in a two by two optical coupling device (optical coupler) 41. Furthermore, an optical wavelength multiplexer 43 is inserted between a port 41b and the optical Kerr medium 42, and an optical wavelength demultiplexer 44 is inserted between a port 41c and the optical Kerr medium 42. A time-division-multiplexed optical signal pulse train which is input via optical circulator 45 is input into port 41a of the optical coupling device 41.
The time-division-multiplexed optical signal pulse train is divided into two by the optical coupling device 41. One of these divided signals is input into the loop from port 41b to propagate clockwise, and the other signal is input into the loop from port 41c to propagate counterclockwise. In addition, a control optical pulse train is input into the loop via the optical wavelength multiplexer 43, propagates clockwise, and is output from the optical wavelength demultiplexer 44 to outside of the loop.
The phase of the time-division-multiplexed optical signal pulse which propagates clockwise, and which is overlapped with the control optical pulse, is changed by the control optical pulse based on the optical Kerr effect. Therefore, the phase of the clockwise time-division-multiplexed optical signal pulse which has been overlapped with the control optical pulse shifts by .pi. rad from the phase of the counterclockwise time-division-multiplexed optical signal pulse when clockwise and counterclockwise time-division-multiplexed optical signal pulses are combined in the optical coupling device 41 again. As a result, the optical signal pulse train 1 which has been overlapped with the control optical pulse is separated and is output from port 41d.
On the other hand, the phase of the clockwise time-division-multiplexed optical signal pulse which has not been overlapped with the control optical pulse almost coincides with the phase of the counterclockwise time-division-multiplexed optical signal pulse. Therefore, the optical signal pulse train 2 is output from the port 41a. This optical signal pulse train 2 is output via optical circulator 45 to the outside. In this way, it is possible to separate only the specified channel that the control optical pulse train is synchronous with in the time-division-multiplexed optical signal pulse train.
A conventional optical pulse demultiplexer having a constitution different from the constitution shown in FIG. 16 will be explained referring to FIGS. 17A and 17B. FIG. 17A shows a conventional optical pulse demultiplexer that utilizes the four-wave mixing effect. The structure shown in FIG. 17A utilizes an optical frequency conversion function by the four-wave mixing effect in an optical fiber (P. A. Andrekson et al., "16 Gb/s all-optical demultiplexing using four-wave-mixing", Elect. Lett. vol. 27, pp. 922-924, 1991). In this device, the time-division-multiplexed optical signal pulse (optical frequency .nu..sub.s) and the control optical pulse (optical frequency .nu..sub.p) are combined by the optical wavelength multiplexer 51, and is input into a third-order nonlinear optical material (polarization-maintaining optical fiber) 52.
In the nonlinear optical material 52, the time-division-multiplexed optical signal pulse and the control optical pulse interact parametrically by the four-wave mixing effect which is one of third-order nonlinear optical effects. Because of the above interactions, in the nonlinear optical material 52, a frequency-converted optical pulse having an optical frequency .nu. (where .nu.=2.nu..sub.s -.nu..sub.p) or a frequency-converted optical pulse having the optical frequency .nu.' (where .nu.'=2.nu..sub.p -.nu..sub.s) is generated. In other words, a frequency-converted optical pulse having an optical frequency .nu. or .nu.' is generated as shown in FIG. 17B for the time-division-multiplexed optical signal pulse overlapping with the control optical pulse. The frequency-converted optical pulse train consisting of such a frequency-converted optical pulse can be separated from the output of the nonlinear optical material 52 using an optical wavelength demultiplexer 53 or an optical filter. In other words, it is possible to separate only the optical signal pulse train of a specified channel that is synchronous with the control optical pulse train from the time-division-multiplexed optical signal pulse train.
An optical pulse demultiplexer different from the optical pulse demultiplexer described above will be explained referring to FIGS. 18A and 18B. FIG. 18A shows the structure of an optical pulse demultiplexer of a polarization-independent type which uses a polarization rotation mirror. The optical pulse demultiplexer shown in this Figure differs greatly from the one shown in FIG. 17A. It utilize the polarization diversity technique in two counter-propagating directions of the loop, and it lets the wavelength of the control optical pulse train coincide with zero dispersion wavelength of the nonlinear optical material (optical fiber) in order to maximize the conversion efficiency, as shown in FIG. 18B. In other words, the structure shown in FIG. 18A is a polarization-independent type in which it is possible for the deimultiplexing operation to be independent of the polarization direction of the input TDM optical signal.
First, the constitution of the polarization rotation mirror MR will be explained. FIG. 19 shows an example of the polarization rotation mirror MR. As shown in FIG. 19, the polarization rotation mirror MR comprises: a polarization beam splitter 56 which reflects the polarization component of the light input from the outside which is in the vertical direction (I.sub.1) with regard to the plane of the paper, and transmits the polarization component in a parallel direction (I.sub.0); a loopwize polarization-maintaing optical fiber 57, the ends of which are connected to the above two output ends of the polarization beam splitter 56 along those of the polarization-maintaing fiber with the principal axes of the polarization.
This polarization-maintaing optical fiber 57 which also works as a nonlinear optical material, is connected with thee two output ends of the above-mentioned polarization beam splitter 56 with one end twisted by 90 degrees around the propagation direction, in order to rotate the polarization direction of light propagating this fiber 57 by 90 degrees. In addition, instead of twisting the polarization-maintaing optical fiber 57 itself, it is possible to rotate the polarization direction of the light in the loop by 90 degrees by using polarization rotating devices such as a Faraday rotator or a half-wave plate. In this way, all the light input to the polarization rotation mirror comes out of the same device with its polarization direction rotated by 90 degrees, instead of the polarization-maintaining optical fibers, it is possible to use other nonlinear optical materials.
In the structure shown in FIG. 18A, the time-division-multiplexed optical signal pulse (optical frequency .nu..sub.s) which is input to the optical wavelength multi-demultiplexer 55 via the optical circulator 54 is combined with the control optical, pulse (optical frequency .nu..sub.p, zero dispersion wavelength of the polarization-maintaing optical. fiber 57), and is input to the polarization rotation mirror MR.
At this time, the polarization direction of the optical signal pulse train is random, and the polarization direction of the control optical pulse train is set at 45 degree angles between two principal axes of the polarization-maintaing optical fiber 57. The optical pulse train input to the polarization beam splitter 56 so the polarization rotation mirror MR is divided into a polarization component which is vertical with regard to the plane of the paper and a polarization component which is parallel with regard to the plane of the paper, and both polarization components clockwise and counterclockwise propagate through the loop consisting of the polarization-maintaing optical fiber 57. The control optical pulse is divided into the two direction, in equal intensity because if the 45 degree polarization, and the optical signal pulse is divided with an arbitrary ratio into the two directions depending on the input polarization state. In the example of FIG. 18, the polarization direction of the component which is propagated counterclockwise is rotated by 90 degrees, and coincides with the polarization direction of the component which is propagated clockwise. Therefore, only one principal axis of the polarization-maintaing optical fiber is used. Because the four-wave mixing conversion efficiency is determined by the control optical pulse intensity, the equal efficiency is achieved in both directions.
The polarization-independent operation is therefore achieved regardless of the fact that the original signal intensity differs in both directions.
The pulse train which is output from the polarization rotation mirror MR is input to the wavelength separation element 58 via the optical wavelength multi-demultiplexer 55 and the optical circulator 54, and is separated into every wavelength. Then, it makes the optical signal pulse train of wavelength .nu..sub.s and the frequency-converted optical pulse train of wavelength .nu.s' spatially separate.
Incidentally, the conventional optical pulse demultiplexer shown in FIG. 16 can only separate a specified channel from other channels with which the control optical pulse is synchronous. Therefore, there is a problem that the circuit of the TDM demultiplexer is complex and large-scale, as a result N-1 optical pulse demultiplexers need to be used to completely separate N TDM channels.
Furthermore, the conventional optical pulse demultiplexer shown in FIGS. 17A and 18A can extract from the TDM optical signal pulse train the frequency-converted optical pulse (.nu., .nu.') of the specified channel to which the control optical pulse is synchronous. Therefore, in order to completely separate N channels, it is necessary to separate one channel per circuit using N optical pulse demultiplexers. Therefore, there is a problem that the circuit is complex and has a large size.